1. Field of the Invention
The present invention generally relates to a method and a system for tuning a scintillation camera having a plurality of photomultiplier tubes by a limited number of reference light sources. More specifically, the present invention is directed to a method for detecting a defect of either reference light sources, or photomultiplier tubes employed in a scintillation camera system, and also to a scintillation camera system capable of performing an automatic defect analysis.
2. Description of the Prior Art
Various types of scintillation camera systems have been widely utilized in the medical electronic field so as to achieve a high-precision nuclear medical diagnosis. A scintillation detector constitutes a major portion of such a scintillation camera system. A typical structure of this scintillation detector is constructed of a scintillator made of a single crystal (NaI: sodium iodide) sandwiched by an aluminum plate functioning as a protector and a glass plate. Furthermore, this scintillation detector is arranged by a light guide and a plurality of photomultiplier tubes. In the scintillation detector, gamma rays emitted from radioisotopes injected into a biological body under medical examination, e.g., a patient, impinge via a collimater onto the single crystal of the scintillator. Thus, the scintillator converts gamma rays into visual light, as scintillation light. The scintillation light is conducted through the light guide to a plurality of photomultiplier tubes. Then, tile scintillation light is converted into a scintillation signal which will then be amplified. The amplified scintillation signal is processed in a detector signal processor, so that positional information and energy information of the impinged gamma rays are analyzed to obtain a desirable nuclear medical image of the patient.
To equalize sensitivities of these photomultiplier tubes which are severely influenced by the environment around this scintillation detector, an output controlling unit has been employed in tile conventional scintillation camera system. In tile output controlling unit, plural reference light sources such as LEDs (light emitting diodes) are provided with a plurality of photomultiplier tubes in such a manner that each LED is positioned adjoining to each photomultiplier. In other words, the total number of LEDs is equal to that of the photomultiplier tubes. To control the output signals of the photomultiplier tubes, the output (detection) signals from the photomultiplier tubes while the LEDs are energized and emit the reference light, are fed back to the detector signal processing circuit in a real time mode. Such a conventional scintillation camera system with the output controlling unit is described in, for instance, U.S. Pat. No. 5,004,904, patented on Apr. 2, 1991, issued to Yamakawa et al.
In tile above-described conventional scintillation camera system, since a large number of reference light sources (equal to the photomultiplier tubes, normally more than 100) are employed to equalize the output signals of the photomultiplier tubes, a quantity of relevant circuit arrangements such as pulsatory light ignition circuits similarly becomes large. Accordingly, complex circuit arrangements and expensive manufacturing costs are required in the conventional scintillation camera system. Furthermore, even when the reference light source becomes defective and thus light emission thereof is lowered, since variations in tile sensitivities of the photomultiplier tubes are different from each other, such a defective phenomenon of tile reference light source cannot be easily detected, resulting in deteriorating reliability of the conventional output control unit.
On the other hand, a total number of reference light sources is reduced, as compared with a total number of photomultiplier tubes, in another conventional scintillation camera system as described in European Patent Application (publication No. 0066768, published on Dec. 15, 1982). In this scintillation camera system, only 11 LEDs are approximately distributed among 37 photomultiplier tubes in such a manner that most of the photomultiplier tubes are continuous to a light emitting diode. Then, automatic amplification control is carried out. However, since there are very severe restrictions about, for instance, tile light emitting characteristics of LEDs with respect to a lapse of time, and also the mechanically stable positions of LEDs, satisfactory sensitivity correction of tile photomultiplier tubes can hardly be realized. In addition, no amplification control for one photomultiplier tube positioned not immediately adjoining to three LEDs separated from each other at an equidistance, is carried out by processing output signals simultaneously derived from these LEDs.